Electrochemical deposition of organic films



EEitifilTiiUN til GRGANLYC Filth 11$ Ernest W. flay cosh, El fierrito, and iionald M. Said,

Richmond, Calih, assignors to hcil Gil tlompany, New

York, N.Y., a corporation of Delaware No Drawing. Fiied Dec. 3% E64, Ser. No. 422,417

a @laims. ll. 264-447) AlBSCT @F THE DZSMESURE This invention relates to electrochemical deposition of a polymer film on a metallic surface to protect it from corrosion and to reduce its current requirements in cathodic protection.

Corrosion protection of metals in various environments is still a troublesome problem, especially in the case of metals exposed to corrosive environments, such as salt water and/or earthen formations. Various coatings have been used on metallic surfaces to protect them against corrosion, but many of these coatings deteriorate in a relatively short time leaving the metallic surfaces exposed to the corrosive environment. Also, surface coatings on metallic surfaces often contain holidays which exposes a small portion of the metallic surfaces through these small openings to the corrosive environment. Corrosion occurring in the holidays tends to undermine the coating and accelerate its deterioration. Thicker coatings will reduce the number of holidays, however they also require substantially greater capital outlays to obtain a holiday-free coating on the metallic structure.

Metallic structures exposed to corrosive environments which are electrolytic in nature, such as salt water and/ or surrounding earthen formations, can also be protected by cathodic protection. Such cathodic protection systems can use sacrificial electrodes (anodes) or apply a direct current through the structure and the corrosive environment to minimize a highly detrimental corrosion that would otherwise occur in such environments without the cathodic protection. Actually, in most cases it is desirable to use both coatings and cathodic protection systems simultaneously since these two protection systems can augment one another in such a situation.

Nothwithstanding whether corrosion protection coatings are used alone or in combination with cathodic protection systems, the deterioration of the coating is a critical factor. Generally, the service life of a coating is directly proportional to its thickness and the thinner the coating, the more likelihood of holidays and a short service life. In any event, coatings on metallic surfaces, especially those surfaces made of ferrous alloys, must have their coatings repaired or replaced at intervals to avoid extremely high corrosion rates. In the past, once the coating has deteriorated to the point that the corrosion rate was unacceptable because of high power requirements limits the economics of the using additional cathodic units, buried structures, such as pipelines, had to be exposed in order to mechanically replace the coating or repair deteriorated portions thereof. Of course, it is very expensive to exhume the pipes, and like structures buried in earthen formations in order to replace or patch the deteriorated coatings. Also, if the coating is attempted to be patched, there is always the danger of the existence of holidays which are not visible and would be overlooked if the coatings were only patched. These holidays provide an active site for corrosive action and results in an undermining action of the coating which seriously limits its service life. Further, when the coating degenerates to the point that high current densities from iaientecl May 28, 1958 "ice cathodic protection systems are required stray current damage becomes a paramount consideration and possibly even more damaging than the corrosion effects.

Because of these problems, a method has been sought by which the coatings could be put on metallic surfaces, renewed or repaired with a reasonable degree of confidence that few, if any, holidays would exist in the coating. Actually, what has been sought Was a in situ method for coating or replacing coatings on immersed or buried metallic structures.

Accordingly, one of the objects of the present invention is to provide a method by which metallic surfaces can be provided with a corrosion protecting coating in situ.

Also, it is an object of the invention, in the broader scope, to provide a method by which organic films can be plated on metallic surfaces electrochemically by polymerizing monomers thereon to provide a film to protect such surfaces from a corrosive environment.

Broadly, this invention encompasses a method for polymerizing olefinic compounds (monomers) onto metallic surfaces by means of electrochemical polymerization to form polymer films on such surfaces. This process is analogous to the electroplating of metals on an electrode surface, except that organic monomers are actually polymerized on or at the electrode surface instead of being plated thereon.

Generally, these compact polymerized films are formed on the electrode's surface by passing a current through an electrolyte containing dissolved olefinic compounds. The mechanism by which the olefinic compounds polymerize on the electrode surface is complex. However, when they are plated on the cathode, the mechanism could be the formation of an hydrogen ion which adds to the olefinic compounds resulting in free radicals. These free radicals induce polymerization on the cathode surface to form a compact polymer film over the electrode surface which continues to grow until the polymerized film completely insulates the electrode from the electrolyte, provided a sufficient concentration of monomer exists in the electrolyte. Alternatively, the mechanism could involve an OH catalyzed reaction on the electrode surface.

By whatever mechanism the polymerization of the olefinic compounds occurs, the polymerization tends to continue as long as current flows through the cathode which will ensure that the cathode surface is substantially encased with the polymer or copolymer film, as the case may be, since any holidays in the film will allow current to flow therethrough and cause further polymerization to occur at such a location, eventually covering the holiday in the film coating. Thus, polymer films applied according to this invention avoid the serious problem experienced when applying coatings With a brush, spray or roller which is the presence of the undesirable holidays. In fact, when applying relatively thin protective coatings, it is not possible to obtain such a holiday-free coating with a brush, spray or roller means of application as one is capable of achieving with the present invention.

The uniform, holiday-free coatings are thin polymer films on metallic surfaces and the method of this invention by which these films are applied has wide flexability, particularly in the protection of buried or metallic structures holding corrosive electrolytes, since a new or replacement polymer film coating can be applied in situ without exposing such structures as would be necessary if a coating was to be mechanically applied with a brush, spray, roller or the like. Specifically, buried metallic structures can be coated by saturating the earth formation surrounding the metallic structure with an electrolyte containing dissolved olefinic compounds (monomers) and then passing a direct current through the electrolyte using the metallic structure as an electrode. Under these circumstances, the olefinic compounds will polymerize on the surfaces of the metallic structure coating any holidays, cracks or breaks or even forming a new film over the entire surface of an uncoated metallic structure.

Since the coating is applied in situ, little trouble is experienced from mechanical abrasion of the coating as wuld be the case if the coating was pre-applied and then the structure was buried. This means that the relatively thin compact polymer films formed according to this invention are extremely serviceable in such situations and have the same serviceability of thicker, more expensive coatings without the high cost thereof.

Besides the protection of buried metallic structure, which is a very attractive feature of the present invention, the application of protective polymer films according to the method of this invention have other broad applications. For example, metallic structures, such as those containing salt water, could also be coated according to this invention in Order to protect them from this corrosive H environment. This could be accomplished by adding enough soluble olefinic compounds to stagnant salt water within the metallic structure to give a concentration of at least 1% by weight adjacent to the metallic structure and then causing current flow through the salt water using the metallic structures to be protected as an electrode. Under these circumstances, the polymer film will form on the metallic structure protecting the surface from further corrosion. This technique could be used in the ballast tanks of ships.

Actually, aqueous electrolytes are useful in the practice of this invention, and in particular, brines are desired electrolytes because of their availability and low cost. Therefore, in most cases, the electrolyte will be an aqueous electrolyte containing salts, such as sodium chloride, or other similarly soluble ionic salts. It has also been found that aqueous gels can be used as the electrolyte, for example aqueous bentonite clay gels in which a water soluble olefinic compound and an electrolytic salt are dissolved.

Olefinic monomers useful in the practice of this invention are, for example, acrolein, acrolein dimer, acrolein trimer, acrylic acid, methyl acrylate, methyl methacrylate and similar polymerizable organic compounds. Since the electrolyte will most often be aqueous, the olefinic compounds useful in the practice of the invention must be at least partially water soluble or can be made so by using organic solvents mixed with water, and this limits these compounds to the l w molecular weight olefinic compounds, similar to those mentioned above. Polymerization, used herein, refers to the formation of polymers or copolymers and the type polymer which is formed depends upon what monomers are dissolved in the aqueous electrolyte. For example, both acrolein and acrylic acid monomers can be added to the electrolyte and copolymerized according to this invention on the electrode surface (the metallic surfaces). Thus, the use of polymers in this specification is intended to include both polymers and cop lymers, and polymerization is meant to cover both polymerization and copolymerization.

Acrolein or its dimer or trimer are especially preferred because of their low cost and effectiveness in low concentrations. Test results have indicated that when acrolein is present in an amount equal to about 1% or greater by weight of the aqueous electrolyte, excellent results can be achieved. Further, it is generally desired that the concentration be from 1 to 2% by weight since this reduces the complete current density required for the best polymerization as will be discussed later. However, greater concentrations are both practical and useful, especially up to by weight.

Generally, direct current is used for the electrochemical polarizati n and, as indicated, most often the metallic surfaces to be coated will be the cathode in the system. Actually, it has been found that in the case of cathod-ically protected buried structures, using direct current protection systems, solutions of electrolyte and olefinic compounds can be flowed around the buried structure and electrochemical polarization will cause film to form on the metal surfaces of the structure.

Further, it has been found that it is desirable where the metal protected has a low hydrogen overvoltage to avoid the evolution of molecular hydrogen at the cathode (the metallic structure to be protected) since this would tend to cause irregularities in the polymer film which are undesirable. To avoid this problem, it has been found that the method can be carried out using pulsed direct current. If the pulse repetition rate is controlled by the cathode potential, for example, a system used to trigger a pulse each time the cathode potential became a 0.9 pulse in a ferrous system with respect to a silver-silver chloride reference electrode, the evolution of molecular hydrogen can be avoided. Therefore, it can be a practice of this invention to use pulsed direct current to effect the polymerization of protective films on the metallic surfaces.

It appears that a good polymerization rate is achieved with current density peaks of approximately to 200 milliamperes per square inch in ferrous systems when using from 1 to 2% by weight of olefinic compounds in the electrolyte. Generally, as the concentration of the acrolein in the electrolyte increases, generally so does the current density peak necessary for the good polymerization. Therefore, at higher concentrations of the acrolein, greater current densities will be desirable. Also, it has been found that when the aqueous electrolyte is basic, the polymerization rate on the cathode surface may be enhanced to some degree.

Having described our invention, the following examples are intended to be illustrative thereof and not a limitation thereon.

EXAMPLE I A 3% sodium sulfate aqueous electrolyte was prepared and 5% by weight of acrolein was added to the electrolyte. Two electrodes were immersed into the solution with the cathode being a ferrous plate. A pulsed direct current having a pulse width of 3.5 milliseconds and a peak current density of milliamperes per square inch was applied through the electrodes immersed in the electrolyte. The pulse repetition rate was controlled by the potential of the cathode and each time the cathode reached the potential of a 0.9 flow with respect to a silver/silver chloride referenced electrode, a pulse was delivered. The system was allowed to operate for a period of two hours after which time the iron cathode was removed from the electrolyte. Inspection of this cathode showed that a compact, regular polymer film had encased the cathode to a substantial degree with a more or less holiday-free protecting film.

EXAMPLE II In this test, which is a broader application of the invention, a simulated oil well casing, which was being cathodically protected with a direct current, was selected for the test. In a two Week test, an aqueous electrolyte containing 1% by weight of acrolein, was flowed around the oil well casing and after approximately two weeks, it was noted that there was a 50% reduction in the current required to maintain the protection potential of a -1.0 volt with respect to a silver/silver chloride referenced electrode. It was also found that a 5% by weight of acrolein in the electrolyte reduced the protection current requirement to /6 of the original value.

EXAMPLE III Ballast tanks were filled with salt water and were cathodically protected by maintaining the system at --1.0 volt with respect to a Ag/AgCl reference electrode. Thereafter, acrolein was added to the salt water until there was a concentration of acrolein in the salt water of approximately 1% by weight.

After about 3 days the current requirements necessary to maintain a 1.0 volt dropped by a factor of 2.5.

It should be appreciated that the types of monomers used to form the film determines its characteristics and also influence the character of the coating films. Since polymers and copolymers can be formed by this process, some selection is available as to the characteristics of the films formed by the instant method. A person skilled in the art can easily evaluate the various films that can be formed according to the method of this invention and can determine What is best suited for any particular application. Clearly this new approach to the protection of buried metallic structures, offers an entirely new tool to cope with the very serious corrosion problem.

We claim as our invention:

1. A method of electrochemically depositing organic protective coatings on a cathodic metallic surface comprising:

(a) dissolving a mono-olefinic monomer selected from the group consisting of acrolein, acrylic acid, methyl acrylate and methyl methacrylate in an aqueous electrolytic brine solution;

(b) contacting the metallic surface to be coated with the resulting solution of said electrolyte and said olefinic monomers; and

(c) passing a direct current through said resulting solution in contact with said metallic surface using said metallic surface as the cathode to effect polymerization of said monomer on said metallic surface to provide a coating thereon.

2. A method according to claim 1 in which the direct current passing through the electrolyte is pulsed to avoid the evolution of molecular hydrogen at the metallic surface.

3. A method of electrochemically depositing organic coatings on a cathodic metallic surface buried in earthen formations comprising:

(a) dissolving at least one mono-olefinic monomer se lected from the group consisting of acrolein, acrylic acid, methyl acrylate and methyl methacrylate in an aqueous electrolytic brine solution;

(b) contacting the buried metallic surface with the resulting solution of said electrolyte and said dissolved monomer by flowing said resulting solution into the earthen formation surrounding said metallic surface; and

(c) passing a direct current through said resulting solution in contact with said buried metallic surface using said surface as the cathode to effect the polymerization of said monomer on said metallic surface to provide a coating thereon.

4-. A method according to claim 3 in which said monomer is a mixture of different mono-olefinic monomers of said group.

5. A method according to claim 3 in which the monomer is acrolein.

6. A method of reducing the power requirements for a cathodically protected metallic surface buried in earthen formations comprising:

(a) dissolving at least 1% by Weight of a mono-olefinic monomer selected from the group consisting of acrolein, acrylic acid, methyl acrylate and methyl methacrylate in an aqueous electrolytic brine solution; and

(b) contacting the buried metallic surface with the resulting solution of said monomer and said electrolyte by flowing said resulting solution into the earthen formations surrounding said metallic surface while said metallic surface is being cathodically protected with direct current.

References Cited UNITED STATES PATENTS 2,726,204 12/1955 Park et al. 204-72 3,070,524 12/1962 Alexander et al. 204147 3,140,276 7/1964 Forster 204-72 3,168,455 2/1965 Shapiro et al. 204148 3,175,964 3/1965 Watanabe et al. 20456 3,201,335 8/1965 MacNab et al. 204-147 3,268,433 8/1966 Abere 204--181 3,304,250 2/1967 Gilchrist 204-181 HOWARD S. WILLIAMS, Primary Examiner.

ROBERT K. MIHALEK, Examiner.

T. TUNG, Assistant Examiner. 

